The present invention relates to the field of detection and location by interferometry, in real time and at a great distance, of lightening strikes, and more particularly it relates to an installation, for such detection and location, which comprises at least two angular locating stations for the strikes, situated at a moderate distance from one another, each station comprising:
at least two fixed receiving antennae for at least one electromagnetic radiation of predetermined wavelength generated by the lightening strikes, and
interferometric processing means for the signals delivered by the antennae, arranged to provide data representative of the elevation and/or azimuth of the source of electromagnetic radiation constituted by the lightening strike.
The known principle of angular location of VHF-UHF sources by interferometry consists of measuring the phase separation .DELTA..phi. between the signals arriving simultaneously at two antennae. It is then possible to find by calculation the angle of incidence .alpha. of the electromagnetic wave with respect to the axis passing through the two antennae by means of the relationship: EQU .DELTA..phi.=2.pi. D/.lambda. sin.alpha.
where D/.lambda. is the distance between antennae expressed in wavelengths.
A measurement of this type enables location of the source in a cone centered on the axis passing through the two antennae and the vertex angel .alpha.. A second interferometric base, that is to say a third antenna (since the same antenna is used as a phase reference), enables the direction of origin of the source to be found, in bearing and in elevation. From the azimuth and elevation data obtained at the two locating stations it is then possible, by calculation of elapsed time, to determine, by triangulation, the position in space of the source.
A known installation for the location in space of lightening strikes comprises two angular locating stations for the strikes, each of the stations comprising two sets of three antennae. The first set is constituted by three antennae separated mutually by a relatively great distance (for example 10 m for an operational frequency of 300 MHZ); it permits precise location of a source but with a large number of ambiguities since the phase measurement is made to the nearest 2.pi.). The second set is constituted by three antennae separated by a relatively short distance (for example 0.5 m) and enables the ambiguities of location of the first system to be removed.
Contrary to all other methods of location which use measurements on the amplitude of the signal (measurements of difference in arrival time, measurements of goniometric type), interferometric location becomes independent of the wave-forms of the lightening signals, by measurements depending only on the phase of the signal. This permits measurements over large energy ranges (typically 80 dB), which is fully adapted to the considerable variations of signal level due to the emission power of the sources, as well as to the considerable extent of the field of surveillance.
However, constructed purely for purposes of study and research on the phenomenology of lightening, this known installation possesses characteristics which do not make it suitable for operational surveillance use enabling the protection of a site to be ensured. In particular, its high resolution in time and in space are superfluous for a surveillance installation; its range is insufficient; its cost is excessive; and especially the processing of the data in elapsed time is incompatable with a surveillance function.
In addition, there is also known, according to Patent Application EP-A-0161940, an installation for the processing of data arranged around a digital computer which determines, by triangulation, the coordinates of the point of impact of a flash with the ground from its angular positions, in azimuth, provided by goniometric locating stations. However, the measurement method used, based on amplitude measurements, and the means employed in each of the stations do not permit the location in space of all the electromagnetic sources associated with a lightening strike. In particular, the locating stations described in this document comprise receiving antennae of the "loop antennae" type which only operate correctly when the magnetic component of the wave to be detected is vertically polarized, which occurs essentially at the striking point of the flash with the ground.